Home now with BGB.Maybe, but if "it is not possible to determine the NON-extraneousness of the water supply pipe" then, per the word of 534.2.6, I don't think you can use it as a protective conductor for supplementary bonding in the manner you have described, since that reg only allows you to do that if it IS extraneous (it doesn't say 'bonded', it says 'extraneous'). If you don't know whether it is extraneous or not then, in terms of the way it's written, you cannot 'enjoy' the provisions of 534.2.6.
543.2.6 but -
I can only suggest that because it is connected to the gas pipe and cpcs it is classed as part of the extraneous pipework in that it may be liable to introduce a potential (not necessarily Earth potential).
Is the gas pipe still extraneous at its boiler connection?
That would be a reason for 544.1.2